--- a/toolkit/components/aboutmemory/content/aboutMemory.js
+++ b/toolkit/components/aboutmemory/content/aboutMemory.js
@@ -13,17 +13,17 @@
 // "file=" argument, and obviously the filename is case-sensitive iff you're on
 // a case-sensitive filesystem.  If you specify more than one "file=" argument,
 // only the first one is used.
 
 "use strict";
 
 // ---------------------------------------------------------------------------
 
-var CC = Components.Constructor;
+let CC = Components.Constructor;
 
 const KIND_NONHEAP           = Ci.nsIMemoryReporter.KIND_NONHEAP;
 const KIND_HEAP              = Ci.nsIMemoryReporter.KIND_HEAP;
 const KIND_OTHER             = Ci.nsIMemoryReporter.KIND_OTHER;
 
 const UNITS_BYTES            = Ci.nsIMemoryReporter.UNITS_BYTES;
 const UNITS_COUNT            = Ci.nsIMemoryReporter.UNITS_COUNT;
 const UNITS_COUNT_CUMULATIVE = Ci.nsIMemoryReporter.UNITS_COUNT_CUMULATIVE;
@@ -43,25 +43,25 @@ XPCOMUtils.defineLazyGetter(this, "nsBin
                                      "setInputStream"));
 XPCOMUtils.defineLazyGetter(this, "nsFile",
                             () => CC("@mozilla.org/file/local;1",
                                      "nsIFile", "initWithPath"));
 XPCOMUtils.defineLazyGetter(this, "nsGzipConverter",
                             () => CC("@mozilla.org/streamconv;1?from=gzip&to=uncompressed",
                                      "nsIStreamConverter"));
 
-var gMgr = Cc["@mozilla.org/memory-reporter-manager;1"]
+let gMgr = Cc["@mozilla.org/memory-reporter-manager;1"]
              .getService(Ci.nsIMemoryReporterManager);
 
 const gPageName = "about:memory";
 document.title = gPageName;
 
 const gUnnamedProcessStr = "Main Process";
 
-var gIsDiff = false;
+let gIsDiff = false;
 
 // ---------------------------------------------------------------------------
 
 // Forward slashes in URLs in paths are represented with backslashes to avoid
 // being mistaken for path separators.  Paths/names where this hasn't been
 // undone are prefixed with "unsafe"; the rest are prefixed with "safe".
 function flipBackslashes(aUnsafeStr) {
   // Save memory by only doing the replacement if it's necessary.
@@ -115,26 +115,26 @@ function debug(x) {
 
 function onUnload() {
 }
 
 // ---------------------------------------------------------------------------
 
 // The <div> holding everything but the header and footer (if they're present).
 // It's what is updated each time the page changes.
-var gMain;
+let gMain;
 
 // The <div> holding the footer.
-var gFooter;
+let gFooter;
 
 // The "verbose" checkbox.
-var gVerbose;
+let gVerbose;
 
 // The "anonymize" checkbox.
-var gAnonymize;
+let gAnonymize;
 
 // Values for the |aFooterAction| argument to updateTitleMainAndFooter.
 const HIDE_FOOTER = 0;
 const SHOW_FOOTER = 1;
 
 // Values for the |aShowTimestamp| argument to updateTitleMainAndFooter.
 const NO_TIMESTAMP = 0;
 const SHOW_TIMESTAMP = 1;
@@ -522,17 +522,17 @@ function updateAboutMemoryFromReporters(
 
   } catch (ex) {
     handleException(ex);
   }
 }
 
 // Increment this if the JSON format changes.
 //
-var gCurrentFileFormatVersion = 1;
+let gCurrentFileFormatVersion = 1;
 
 
 /**
  * Parse a string as JSON and extract the |memory_report| property if it has
  * one, which indicates the string is from a crash dump.
  *
  * @param aStr
  *        The string.
@@ -696,17 +696,17 @@ function updateAboutMemoryFromTwoFiles(a
       }
     });
   });
 }
 
 // ---------------------------------------------------------------------------
 
 // Something unlikely to appear in a process name.
-var kProcessPathSep = "^:^:^";
+let kProcessPathSep = "^:^:^";
 
 // Short for "diff report".
 function DReport(aKind, aUnits, aAmount, aDescription, aNMerged, aPresence) {
   this._kind = aKind;
   this._units = aUnits;
   this._amount = aAmount;
   this._description = aDescription;
   this._nMerged = aNMerged;
@@ -1341,17 +1341,17 @@ function sortTreeAndInsertAggregateNodes
   // is significant;  there's no point creating an aggregate node that only has
   // one child.  Just process it.
   sortTreeAndInsertAggregateNodes(aTotalBytes, aT._kids[i]);
 }
 
 // Global variable indicating if we've seen any invalid values for this
 // process;  it holds the unsafePaths of any such reports.  It is reset for
 // each new process.
-var gUnsafePathsWithInvalidValuesForThisProcess = [];
+let gUnsafePathsWithInvalidValuesForThisProcess = [];
 
 function appendWarningElements(aP, aHasKnownHeapAllocated,
                                aHasMozMallocUsableSize) {
   if (!aHasKnownHeapAllocated && !aHasMozMallocUsableSize) {
     appendElementWithText(aP, "p", "",
       "WARNING: the 'heap-allocated' memory reporter and the " +
       "moz_malloc_usable_size() function do not work for this platform " +
       "and/or configuration.  This means that 'heap-unclassified' is not " +
@@ -1681,17 +1681,17 @@ function appendMrNameSpan(aP, aDescripti
 
 // This is used to record the (safe) IDs of which sub-trees have been manually
 // expanded (marked as true) and collapsed (marked as false).  It's used to
 // replicate the collapsed/expanded state when the page is updated.  It can end
 // up holding IDs of nodes that no longer exist, e.g. for compartments that
 // have been closed.  This doesn't seem like a big deal, because the number is
 // limited by the number of entries the user has changed from their original
 // state.
-var gShowSubtreesBySafeTreeId = {};
+let gShowSubtreesBySafeTreeId = {};
 
 function assertClassListContains(e, className) {
   assert(e, "undefined " + className);
   assert(e.classList.contains(className), "classname isn't " + className);
 }
 
 function toggle(aEvent) {
   // This relies on each line being a span that contains at least four spans: